Sigmoidally hydrochromic molecular porous crystal with rotatable dendrons

Abstract

Vapochromic behaviour of porous crystals is beneficial for facile and rapid detection of gaseous molecules without electricity. Toward this end, tailored molecular designs have been established for metal-organic, covalent-bonded and hydrogen-bonded frameworks. Here, we explore the hydrochromic chemistry of a van der Waals (VDW) porous crystal. The VDW porous crystal VPC-1 is formed from a novel aromatic dendrimer having a dibenzophenazine core and multibranched carbazole dendrons. Although the constituent molecules are connected via VDW forces, VPC-1 maintains its structural integrity even after desolvation. VPC-1 exhibits reversible colour changes upon uptake/release of water molecules due to the charge transfer character of the constituent dendrimer. Detailed structural analyses reveal that the outermost carbazole units alone are mobile in the crystal and twist simultaneously in response to water vapour. Thermodynamic analysis suggests that the sigmoidal water sorption is induced by the affinity alternation of the pore surface from hydrophobic to hydrophilic.

Fig. 1. Schematic representations of the hydrochromism of VPC-1 upon hydration/dehydration.

a Molecular structure of a second-generation dendrimer 1 with two branched Cz dendrons and a DBPHZ core. b Photographs of VPC-1^red and VPC-1^yellow together with schematic illustrations of the microporous crystalline grains that change their colour in response to H₂O vapour.

Fig. 2. ORTEP diagrams of 1 and characterizations of VPC-1.

a An ORTEP diagram of one of the four crystallographically non-equivalent molecules of 1 with a probability level of 30%. Pink arrows indicate the twisting of the internal Cz unit with respect to the DBPHZ core and the external Cz unit with dihedral angles θ₁ and θ₂, respectively. b A side-view of one of the four crystallographically non-equivalent molecules of 1 with a probability level of 30%. c Humidity-dependent diffuse reflectance spectra of VPC-1 upon increasing the surrounding humidity from 3.9 to 84.3%. d Plots of the K–M values at 570 nm upon increasing and decreasing the relative humidity. e Powder X-ray diffraction profiles of VPC-1^red (red), VPC-1^yellow (orange), and as-synthesised amorphous yellow solid (grey). f N₂ adsorption isotherm of VPC-1 at −196 °C.

Fig. 3. Spectroscopic data and computational molecular models for the simultaneous twisting of the external Cz units in response to H₂O uptake.

a–d FTIR spectra of VPC-1 on increasing the relative humidity at 25 °C. Selected spectral regions including vibrational bands attributed to C–H bending (b), C–N stretching, pyrrole breathing and C–H bending (c) and O–H stretching modes (d) are magnified for clarity. e, f Proposed molecular structures of 1 in VPC-1^yellow (e) and VPC-1^red (f), which are computationally modelled based on the FTIR, Raman and diffuse reflectance spectra. g Changes in the transmittance of the C–H bending (open circles) at 1142 cm^–1 and O–H stretching (closed circles) vibrational bands at 3448 cm^–1 upon increasing RH from 20.8 to 80.9%.

Fig. 4. H₂O sorption into VPC-1.

a–c H₂O adsorption (closed circles)/desorption (open circles) isotherms of VPC-1 measured at 10 (a), 20 (b), and 30 °C (c). d Isosteric heat of adsorption (Q_st) for H₂O plotted against the H₂O uptake into VPC-1. The upper horizontal axis shows the molar ratio of the H₂O uptake to 1. Yellow- and red-coloured regions indicate hydrophobic and hydrophilic pore surfaces of VPC-1, respectively.